1. Field of the Invention
The invention concerns a process for the manufacture of flexible polyurethane foams consisting of a mixture of diphenylmethane diisocyanates and polyphenylene polymethylene polyisocyanates (crude MDI) having a functionality greater than 2 containing 55 to 85 percent by weight diphenylmethane diisocyanate, polyester polyols or mixtures of polyester polyols and polyether polyols and auxiliaries and additives having a high resistance to hydrolysis, high load-bearing capacity, and high energy absorption upon impact.
2. Description of the Prior Art
The manufacture of flexible polyurethane foam is a known process. Toluene diisocyanate and particularly, the commercially available toluene diisocyanate isomer mixtures are commonly used as polyisocyanates. The disadvantage is that toluene diisocyanates, due to their high vapor pressure, are relatively strongly toxic and therefore, special precautionary measures must be taken and observed during processing.
In order to reduce the toxicity hazard and increase the reactivity, it has been suggested to replace the toluene diisocyanates with mixtures of toluene diisocyanates and a mixture of homologous polyarylene polyalkylene polyisocyanates for the manufacture of polyurethane plastics, including foams.
According to British Pat. No. 874,430, flexible polyurethane foams are also produced by reaction of polyether polyols with at least two hydroxyl groups and a polyisocyanate mixture consisting of diarylmethane diisocyanates and 5 to 10 percent by weight of a polyisocyanate having a functionality greater than 2 in the presence of water. According to German Published Application, No. 24 25 657, diphenylmethane diisocyanate mixtures having an average isocyanate functionality of less than 2.4 and not containing more than 60 percent by weight of 4,4'-diphenylmethane diisocyanate, with at least 15 percent by weight of the diphenylmethane diisocyanate isomer mixture consisting of 2,4'-diphenylmethane diisocyanate, are suited for the manufacture of integral skin foams.
All of these polyurethane foams, which are of great elasticity, have in common one drawback which prohibits their use in certain areas of application. This is their insufficient load-bearing capacity and their deficient energy absorption upon impact, particularly in the case of flexible foams of low densities. In order to lessen this drawback, it has been suggested to increase the density. This measure, however, only resulted in increasing the cost per piece. A satisfactory improvement of the above-mentioned mechanical properties could not be achieved with these measures.